Abstracts

Neural activity can be modulated with low-frequency ([lt]100Hz) electric fields. We are currently working to implement neural prosthetics for In Vivo applications, especially seizure control, based on this technology. In order to modulate activity in chronically implanted animals with electric fields, high charge density passing stimulation electrodes are required, where current is primarily limited by electrochemistry at the electrode surfaces. Here we evaluate performance and safety of electrodeposited iridium-oxide electrodes in chronically implanted and stimulated animals, and compare results to two In Vitro models. Male Sprague-Dawley rats were bilaterally implanted with hippocampal stimulating (3mm long x 0.25mm diameter) and recording electrodes. Electrochemical deposition of iridium oxide film was used on all electrodes. Six days after surgery interhemispheric stimulation was applied for 100 h continuously, with 0.6V (voltage controlled) and frequency from 0.1 to 50Hz. Rats were then sacrificed, perfused and histology performed. Electrode charge storage capacity was measured by cyclic voltammetry and impedance spectroscopy. In order to mimic the In Vivo situation, electrodes were immersed in 0.9%NaCl and submitted to the same stimulation protocol as described above. Charge passed per stimulation cycle as a function of frequency was calculated (Fig.1, M1) by integrating the electric current through the electrodes over each period of the stimulating waveform. Bipolar stimulation In Vivo yielded lower charge per cycle than In Vitro (Fig.1, R1 and R2). The In Vitro model was then adapted by adding an 8.2k[Omega] resistor in series with the electrodes to model higher net impedance between the electrodes. This model (Fig.1, M2) showed a better fit to the In Vivo data at higher frequencies. Electrochemical analysis of charge passed through implanted electrodes with the voltage control stimulation protocol showed that the net impedance through bilateral stimulation In Vivo was higher than expected. This limits the amount of current that can be safely passed in deep brain stimulation paradigms, and can be addressed with ipsilateral in contrast with bilateral electrode pairs. These results are relevant for safety assessment, optimal performance, and design of controls for high charge passing stimulation electrodes.[figure1] (Supported by NIH grants R01EB001507, K02MH01493, and R01MH50006.)